FN SLP or its American brother, Winchester SX3. I love my SLP, as it is super reliable. As for the whole having two pistons thing, I haven't even used the "light" piston yet. After a whole case of low-brass game loads, it may short stroke every 20th round or so, and that's with the heavy piston, which is meant to be cycled by 3" magnum loads. Also, I've never had a jam that wasn't easily cleared just by pulling back the bolt all the way or hitting the slide release button.
fastest cycling semi 12 on the market
If you want to see how fast it can really go, watch the Patrick Flanigan Winchester SX3 promotional video; he puts out 12 rounds in 1.442 seconds